The application of sedimentary microfacies on the fracability of tight sandstone reservoir in Chang 7 member of Longdong area in the Ordos Basin
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摘要: 沉积特征差异是控制储层非均质性的关键因素之一,利用沉积微相来分析储层的非均质性通常是油气田开发和储层甜点预测的重要手段,也可以尝试用其评价致密砂岩储层的可压裂性。以鄂尔多斯盆地陇东地区延长组7段(长7段)的致密砂岩为研究对象,在通过岩芯、测井等资料识别不同沉积微相类型基础上,利用全岩X射线衍射(XRD)分析、铸体薄片观察得到致密砂岩样品的矿物成分和结构参数,并结合岩石力学实验,采用脆性指数和三轴抗压强度的比值来表征岩石的可压裂性。将致密砂岩的沉积微相、成分结构和可压裂性进行对比分析后得到以下认识:长7段致密砂岩主要发育水下分流河道和席状砂2种沉积微相,不同沉积微相之间的矿物成分、结构存在明显差异;与席状砂相比,水下分流河道砂体的平均粒径更大,分选更好,碳酸盐矿物和黏土矿物含量更低,杂基含量更少;可压裂性指数与砂岩的石英含量、碳酸盐矿物含量、粒度分布标准偏差具有正相关性,与长石含量、平均粒径具有负相关性。灰色关联分析表明碳酸盐矿物含量、粒度分布标准偏差、粒径是影响长7段致密砂岩可压裂性的最主要因素,整体上席状砂的可压裂性指数要高于水下分流河道砂,更易于压裂。由于砂岩颗粒的粒度分布标准偏差、粒径受沉积微相控制,碳酸盐矿物含量受砂岩厚度直接控制、受沉积微相间接控制,故在致密砂岩油气储层压裂的实际工程中可以依据沉积微相差异来判断致密砂岩的可压裂性变化,简化可压裂性的评价流程。Abstract:
Objective Sedimentary differences are the key factor in controlling reservoir heterogeneity. Analyzing reservoir heterogeneity through sedimentary microfacies is crucial for oil and gas field development and sweet spot prediction, and it also informs the evaluation of fracturing in tight sandstone reservoirs. There are many types and complex lithologies of unconventional oil and gas reservoirs in the Ordos Basin as well as many factors controlling reservoir fracability. At present, mechanical experiments are used to comprehensively characterize the fracturing property; however the research cost is high and the experimental process is complicated, making it unnsuitable for large-scale oilfield development and use. Therefore, this study attempted to analyze and compare the fracability of tight sandstones with different sedimentary microfacies from the perspective of sedimentary microfacies controlling the lithology and reservoir development to provide a reference for oilfield development plans. Methods Taking the compact sandstone of Chang 7 member of Yanchang Formation in the Longdong area of Ordos Basin as the research object, the different types of microfacies are identified through the data of core and cast slice, the mineral composition and structural parameters of rock samples were obtained by X-ray diffraction (XRD) analysis, and rock mechanics experiments were conducted to quantitatively described the fracturing property. Results The results are as follows: (1) Two sedimentary microfacies, namely underwater distributary channel and sheet sand, mainly developed in Chang 7 Member of Yanchang Formation in the study area. Among them, the single sand body thickness of the underwater distributary channel is greater than 2 m, the sheet sand is mostly a medium thin and thick sand mudstone interlayer, and the single sand body thickness is generally less than 2 m. (2) The composition and structure of the two sedimentary sandstone microfacies are obviously different: the content of carbonate minerals, clay minerals, and heterobases in the sheet sand microfacies are relatively high, the particle size is finer, and the sorting is worse, which are the main internal factors that cause the difference in tight sandstone fracability and are the basis for judging the fracability of tight sandstone by sedimentary microfacies. (3) The fracability index was related to the composition and structure of sandstone. In terms of composition, the fracability index was positively correlated with quartz mineral content as well as carbonate mineral content and negatively correlated with feldspar mineral content. In terms of structure, there is a negative correlation between the fracability index and the average particle diameter φ. The larger the particle size, the higher the fracability index. The fracability index was positively correlated with the standard deviation of the particle size, indicating that the worse the particle separation, the higher the fracability index. (4) Through grey correlation analysis, it was found that the degree of influence of sandstone parameters on fracability was in the order of carbonate mineral content, quartz content, standard deviation of particle size, and average particle size from high to low, while clay minerals and feldspar content were in a relatively weak position. Conclusion The results indicate that the higher the contents of carbonate and quartz and the higher the standard deviation of particle size (the worse the sorting), the better the fracability. The finer the particle size, the higher the feldspar content and the worse the fracability. Grey correlation analysis showed that the carbonate mineral content, separation and particle size play a major role in the fracability of tight sandstone. Compared with distributary channel microfacies, sheet sand has a higher carbonate minerals content, worse sorting, and little difference in quartz content. Although the microphase particle size of the sheet sand is slightly finer, the average particle size has a relatively minor effect on the fracability; thus, sheet sand as a whole shows better fracability. Significance Since the standard deviation (sorting) and particle size of sandstone particles are controlled by sedimentary microfacies, and the content of carbonate minerals is directly controlled by sandstone thickness and indirectly affected by sedimentary microfacies, the change in the tight sandstone fracability index can be judged according to the difference in sedimentary microfacies in practical engineering, and the working process can be simplified. -
Key words:
- Yanchang Formation /
- tight sandstone /
- fracability /
- sedimentary microfacies /
- distributary channel /
- sheet sand
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图 2 不同沉积微相砂岩样品的显微镜下特征
Figure 2. The characteristics of different sandstone microfacies under the microscope
(a) Underwater distributary channel microfacies of sandstone, with good sorting, sub-angular, well Y24 at 2150.7 m, under monochromatic polarized light; (b) Sheet sand microfacies of fine sandstone, poor sorting, sub-angular, well Y24 at 2141.6 m, under monochromatic polarized light
图 3 岩芯中典型的沉积相标志
Figure 3. Typical sedimentary phenomena in rock cores
(a) Fluvial cross-bedding, well Y16 at 2040.7 m; (b) Flaser bedding, well Y24 at 2145.2 m; (c) Intact carbon debris in sandstone, well Y16 at 2026.7 m; (d) Disc-shaped mud chips in sandstone with oxidation halo, well Y24 at 2143.8 m; (e) Muddy rip-up clasts in sandstone, well Y24 at 2150.7 m; (f) Flame structure at the base of sandstone, well Y24 at 2161.8 m
表 1 陇东地区延长组地层划分及岩性特征
Table 1. Stratigraphic division and lithological characteristics of the Yanchang Formation in the Longdong region
地层时代 地层岩性 统 组 段 上三叠统 延长组 1 深灰色泥、页岩夹煤层,局部为厚层块砂 2 灰绿色中—细砂岩夹灰色—深灰色泥岩、黑褐色炭质泥岩 3 4+5 深灰色—灰黑色泥、页岩与灰色—灰绿色粉砂岩互层,下部发育一套油页岩、泥岩夹薄层凝灰岩 6 7 8 深灰色—灰黑色泥岩与浅灰绿色—褐灰色中—细砂岩互层 9 10 灰绿色厚层块中—粗长石砂岩夹深灰色及暗紫色泥岩 -
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